1. Field of the Invention
The invention relates to filtration during polymer melt spinning, and in particular to porous filters for use in spin pack assemblies.
2. Description of Related Art
Synthetic polymer fibers typically are manufactured by extruding filaments of molten polymer under pressure through openings in plates called “spinnerettes,” which are contained in “spinnerette heads” in spinning units known as “spin packs.” Before extrusion through the spinnerette, the polymer melt must be filtered to remove solid contaminants and gelled polymer particles. Unless removed by filtration, such impurities can clog the spinnerette or pass through the spinnerette and cause defects in the product polymer fiber.
Various filtration systems have been used in spin packs to filter the polymer melt immediately prior to extrusion through the spinnerette. Ideally, the filtration media should retain particulate impurities and also impart shear, i.e., induce alignment and reduce cross-linking between polymer chains. Known filtration media include sand, shattered metal, metal fiber, screen packs, and porous metal discs and cups. A traditional spin pack filtration assembly includes loose filtration media, such as sand or shattered metal. The loose fill is assembled in situ at the site of polymer filtration as a layered bed confined between screens, and a sealing ring is used to seal the filtration assembly within the spin pack and prevent polymer leakage. The bed of loose fill generally includes multiple layers of particles, with each layer having progressively finer particle size. These layers create a depth filtration effect, which prolongs the life of the filter because larger contaminant particles are removed by the coarse upstream filtration layers, leaving the finer downstream filtration layers open to retain smaller contaminant particles. However, loose media do not provide optimal filtration, as they tend to migrate, separate, channel, and fluidize. Such irregular, uncontrollable motion of the particles of loose fill reduces filtration effectiveness and causes inconsistent filtration over the life of a filter and across filters. Similarly to traditional loose fill, metal fiber has a soft, weak structure that must be surrounded by screens to prevent migration under pressure. Further, metal fiber has a large open void volume, which affords great dirt holding capacity but has limited ability to impart shear.
Porous metal discs and cups have a fixed, sintered structure that provides good shear and affords controlled, consistent filtration because it is not subject to migration. However, traditional porous metal filters often have difficulty withstanding the high pressures used in polymer melt extrusion or, if they are thick enough to withstand such pressures, afford sub-optimal flow rates. Furthermore, porous metal discs and cups often suffer from reduced filtration life due to surface blinding and caking.
To reduce the pressure drop across the filter and improve filtration life, sintered metal filters having extended filter surface area have been made. Some such extended area filters include cylindrical or conical cavities defined by multiple distinct tubular filter elements (e.g., Mott, U.S. Pat. No. 3,570,059) or an integral cavity-containing structure (e.g., Bergstrom, U.S. Pat. Nos. 3,746,642 and 3,788,486). Such filters offer extended filtration area, but sometimes include a multi-component assembly (e.g., a group of cups in an adapter) that is subject to leakage between components. Furthermore, many extended area filters require a thick inter-cavity wall structure to afford sufficient strength for high-pressure applications, which adversely affects flow rate and throughput. In addition, surrounding support structure (e.g., breaker plate and screens) is often required to prevent the filter from bending, fracturing, or collapsing under pressure.
Furthermore, the production of many extended area filters is time-consuming and expensive, and some commonly used production steps cause shortcomings in the end product. For example, machining steps typically used to form the cavity structure of extended area filters often cause distortions in the pores and surface morphology of the filters, such as non-uniform density, smeared pores, and surface blinding. Such structural distortions result in reduced flow rate and consistency, and decreased filtration life. Another common production technique that causes drawbacks in the final filter product involves the use of polymeric binders. Extended area filters are often made from a dispersion of metal powder mixed with a binder. The use of such a dispersion can adversely affect the retention rating of the final filter product, e.g., due to non-uniformity of the dispersion, shear and damage to the metal particulate during mixing, and shelf life limitations of the dispersion. Moreover, the binder is later burned off from the final filter product, leaving behind polymer binder decomposition products, e.g., residual carbon, as contaminants that affect the corrosion resistance and surface chemistry of the filter.
Thus, a need remains in the art for new extended area filters, and methods of making the same, that provide controlled, consistent filtration with good flow rate and filtration life, and can be efficiently and cost-effectively manufactured, installed, serviced, and replaced.